Ab initio calculations for para-substituted phenylnitrenium ions predict larger singlet-triplet splittings, shorter singlet C-N(+)bond lengths, and higher singlet aromatic ring Stretching frequencies for substituents with greater electron-donating character. Trends in these properties correlate linearly with para-substituent constants sigma(R)(+), indicating that phenylnitrenium ions closely resemble other electron-deficient aromatic systems where resonance interactions with substituents are dominant. Sensitivity to substitution is large as judged by the slope of the correlating line for aqueous singlet-triplet splittings, rho = 6.4. For 13 of 15 substituted cases, aqueous solvation preferentially stabilizes the singlet state by 0.9 to 4.4 kcal/mol; for;the p-CO2H and p-CF3 cases, the triplet state is better solvated by less than 1 kcal/mol.